The present invention relates to an image sensing method and apparatus and, more particularly, to a dynamic range expansion method for a high-resolution image sensed by an image sensing apparatus, such as a digital still camera.
Recently, quality of an image sensed by a digital still camera has been increased. This progress is greatly due to the improvement in resolution of a CCD (charge-coupled device) sensor, and decrease in price of memory. However, the resolution of a CCD is approaching a limit when it is attempted to increase the quality of an image only by increasing the resolution of the CCD. A CCD sensor having about 400,000 pixels is currently most popular, whereas, a currently demanded resolution is 1,500,000 pixels, and even further, 6,000,000 pixels. If it is attempted to realize such a high-resolution CCD sensor in a form of a CCD area sensor, the size of each pixel becomes very small, which causes deterioration in sensitivity and saturation level (i.e., dynamic range) drop.
Accordingly, as a method of improving the resolution of an image without increasing the number of pixels in a CCD sensor, xe2x80x9cpixel shiftingxe2x80x9d technique as disclosed in the Japanese Patent Publication No. 50-17134 is known. In the pixel shifting method, a plurality of images sensed by a plurality of CCDs which are placed at different positions separated by a predetermined amount are combined to generate a single image of high resolution. Further, a method of combining a plurality of images sensed by a single CCD while shifting its position by a predetermined amount to obtain a single image of high resolution, such as the one disclosed in the Japanese Patent Publication No. 1-863, is also known.
Furthermore, as a method for obtaining an image of wide dynamic range without increasing the dynamic range of a CCD sensor, a method of sensing a plurality of images while changing luminous exposures and combining them, as disclosed in the Japanese Patent Application Laid-Open No. 59-54384, is known.
In the aforesaid conventional examples, however, the above two techniques involving pixel shifting processing and dynamic range expansion processing are written in different documents, and there is no discussion on integrated both technique. Therefore, when these two techniques are independently adopted for sensing an image of an object in order to improve quality of the image, there is a problem in which a huge memory area in a memory or memories is required.
For example, assuming the use of a single CCD of 1,500,000 pixels to obtain a dynamic-range-expanded image of 6,000,000 pixels, first, total of four images are to be sensed at a first position (reference position) and three other positions where the CCD is shifted in the vertical, horizontal, and oblique directions with respect to the reference position. Then, another four images for expanding dynamic range are to be sensed at a different luminous exposure at the above four positions. In other words, two images of 1,500,000 pixels has to be sensed in different luminous exposures at each of four positions, namely, eight (=4xc3x972) images are to be sensed. If image data is analog-digital converted in depth of ten bits, then,
1,500,000(pixels)xc3x9710(bits)/8(bits)xc3x978(images) =15(MB)
Thus, a very large memory area is needed.
The present invention has been made in consideration of the above situation, and has as its object to obtain a dynamic-range-expanded image of high resolution while consuming a smaller memory area without increasing the number of pixels of an image sensing device, such as a CCD.
According to the present invention, the foregoing object is attained by providing an image sensing method comprising: an image sensing step of obtaining image data of a plurality of images of an object in a plurality of different luminous exposures; a dynamic range expansion step of combining the image data obtained in the image sensing step and generating image data of a single image of expanded dynamic range; a shifting step of shifting an image formation position of the images of the object on an image sensing device to one or more positions; a repeating step of repeating the image sensing step and the dynamic range expansion step at each image formation position shifted by the shifting step and generating image data of a plurality of images of expanded dynamic range; and a combining step of combining the plurality of images of expanded dynamic range obtained in the repeating step and generating image data of a single image of high resolution.
According to the present invention, the foregoing object is also attained by providing an image sensing apparatus comprising: image sensing means for sensing a plurality of images of an object in a plurality of different luminous exposures and outputting the image data of the plurality of images; first storage means for storing the image data of the plurality of images of the object sensed by the image sensing means in the plurality of different luminous exposures; dynamic range expansion means for combining the image data stored in the first storage means and generating image data of a single image of expanded dynamic range; second storage means for storing image data of a plurality of images obtained by the dynamic range expansion means; shifting means for shifting an image formation position of an image of the object on the image sensing means to one or more positions; and combining means for combining the image data of the plurality of images stored in the second storage means and generating image data of a single image of high resolution.